[RTE-513] Bugfix: Ensure sleep takes long enough on SGX platform

[raoulstrackx]

Summary
The thread::sleep function documentation states:

Puts the current thread to sleep for at least the specified amount of time.

And for thread::sleep_until:

Puts the current thread to sleep until the specified deadline has passed.

These did not hold for the x86_64-fortanix-unknown-sgx platform. This PR fixes this issue.

More details
The attack model on Intel SGX states that enclaves should not rely on timing to be accurate within an enclave. To ensure that users do not rely on the accuracy of the wait usercall (see the ABI), we randomized the specified timeout with +/-10%. This was causing issues when the timeout was reduced. This PR fixes the issue by picking a random duration of up to +10% (i.e., the duration will never be reduced).
The issue was caught when #141829 added a test for sleep_until. There wasn't a test in the standard library to verify that thread::sleep sleeps for at least the specified amount of time. Such a test is added as well.

cc: @jethrogb @aditijannu

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